The present invention relates to the use of a peelable mask as a sealant against potting compound.
When soldering a printed circuit board (PCB) in a flow solder machine or a solder bath, solder adheres to any exposed metal surfaces it comes in contact with.
Commonly, a peelable solder-repellent mask is used to prevent unwanted solder adherence, also known as solder xe2x80x9cpick upxe2x80x9d. The peelable solder mask is arranged to cover areas of exposed metal where solder adherence would be undesirable for manufacturing or safety reasons and as the name suggests this mask can then be pealed away to reveal unsoldered metal areas. Materials used as solder masks include epoxy and acrylate compounds.
Manufacture of electronic devices often involves a plurality of assembly stages, including one or more soldering stages. In the manufacture of electronics whose components are attached to a PCB from both sides, it is desirable that the components for a first side of the PCB be soldered in place before components for a second side are attached. Soldering components to the first side of the PCB means exposing the second side to solder. The second side may also be provided with metal contacts to which components have yet to be attached, for example pressure contacts for components which will be mechanically attached to the second side after soldering. A peelable mask may be used to prevent solder pick up on these metal contacts. The metal contacts on the second side are masked while components are soldered to the first side then the mask is removed to allow components being attached to the second side of the PCB to make contact with the PCB itself.
One example of electronics where components are attached to a PCB from both sides is the electronics for an inverter for a COMBIMASTER [Registered Trademark] CM411 combined inverter and motor. It will be understood however that the following discussion relates not only to this specific example but to the attachment of electrical components to PCBs in general.
In many electrical devices, including inverters, physical factors can degrade the performance of electronics. Vibration, heat, humidity and proximity of components are all problems which afflict electronic circuits. To ameliorate these afflictions, it is advantageous to immerse electronic circuits in a potting compound, for example a urethane or epoxy resin. Potting compound reduces vibration of components and protects against impact and shock; protects against static discharge by having a low dielectric constant in comparison to air; excludes the ingress of moisture, dust particles and chemicals; has a higher heat conductivity than air, thus giving a steeper temperature gradient with distance from components; and for all these reasons, improves a circuit""s tolerance of component proximity.
Alternatives to potting compounds have their concomitant drawbacks, conformal coating can be applied either by spraying or dipping. Conformal coating does exclude moisture but vibration is not reduced to the same extent as in potting compound and the thermal conductivity remains virtually unchanged over air. More conclusively, conformal coating is costly.
Potting compound, though desirable in some parts of an electronic device can impair the performance of other parts. To exclude potting compound from the latter parts, the electronic device is provided with mechanical seals or gaskets.
In the example of the electronics for an inverter, the assembly is further complicated by the requirement that one modular component must be held in place by compression, rather than soldering, between the modular component and a PCB. The modular component makes electrical contact to the PCB by means of compressible conductor legs. Not only is the modular component a component of the type that potting compound can render unusable but the introduction of a conventional gasket to prevent the ingress of potting compound renders the compressible conductor legs, and therefore the whole modular component, disabled.
An object of the present invention seeks to seal the electrical connections of a modular electrical component (in terms of the example, a Semikron MiniSKiiP [RTM] Module) from the ingress of a potting compound when it is mechanically attached to a printed circuit board.
It is therefore an object of the invention to obviate or at least mitigate the aforementioned problems.
In accordance with one aspect of the present invention, there is provided a method for assembling an electrical device having a printed circuit board, the printed circuit board having a first side and a second side and being provided with slots for receiving conducting legs of at least one electrical component, the method including the steps of:
a) masking an area of the second side of the printed circuit board with a solder-repellent mask, the masked area comprising a first portion and a second portion;
b) disposing said at least one electrical component on the first side of the printed circuit board with the conducting legs extending through the slots from the first side to the second side;
c) fixing said at least one electrical component in place by exposing the second side of the printed circuit board to solder and allowing the solder to solidify;
the method characterised by further including the steps of:
d) removing the solder-repellent mask from the first portion to expose at least one contact while leaving the solder-repellent mask in place over the second portion; and
e) disposing a further electrical component directly against the second side, a lower surface of the further electrical component lying adjacent to the second side, the second portion corresponding to at least those points of the masked area at which the lower surface of the further electrical component touch the masked area, the second portion thereby forming a seal between the further electrical component and the second side.
The method preferably further includes the step of:
f) submerging the electrical device in a potting compound, the second portion preventing the ingress of potting compound between the further electrical component and the printed circuit board.
The masked area may be a single contiguous area.
The second portion may be an outer portion of the masked area and the first portion, an inner portion of the masked area, such that all points on the periphery of the masked area lie within the second portion.
The exposure to solder in step c) is preferably achieved through a flow solder technique.
The further electrical component can be an inverter module.
In a further aspect of the present invention there is provided a peelable mask for masking a printed circuit board from solder pick up, when solder is applied to the printed circuit board and for sealing a gap between an electronic component and the printed circuit board from the ingress of a potting compound.
The peelable mask may cover a masked area having an inner portion and an outer portion, the mask covering the inner portion being removed subsequent to soldering.
Thus according to the present invention, a peelable mask can be used to protect an area of a printed circuit board (PCB) provided with contacts for a modular electrical component, for example a MiniSKiiP Module, from exposure to solder. Furthermore, at least part of the peelable mask can be used as a mechanical seal to stop the ingress of potting compound between the modular electrical component and the PCB.